A Japanese university developed an electric double layer capacitor (EDLC) dubbed the "Nanohybrid Capacitor," which has an energy density per unit volume of 20Wh/l.

According to the Graduate School of the Tokyo University of Agriculture and Technology (TUAT), the new capacitor has three times as high energy density as existing EDLCs. When commercialized, it may be utilized to store renewable energy or as regeneration systems for hybrid cars, etc.

Like the existing EDLCs, it uses activated carbon for the positive electrode. But lithium titanate (Li4Ti5O12) is used for the negative electrode to enhance energy density. In addition, TUAT formed nanocrystallized Li4Ti5O12 with a particle diameter of about 5-20nm and mounted it on carbon nanofibers so that the time required for charging/discharging becomes shorter than that with existing EDLCs.

Charge/Discharge in Only 12 Seconds

In the new capacitor, lithium (Li) ions and electrons are absorbed on the negative electrode during charging, and then dispersed inside the electrode to be turned into Li7Ti5O12. During discharging, electrons are released by reverse reaction.

There have been reports of EDLCs that use Li4Ti5O12 for the negative electrode to improve the energy density. But such EDLCs are faced with a problem of long charging/discharging time due to an extremely low output density resulting from a low electric conductivity of Li4Ti5O12 and a low diffusion constant of Li ions that impedes the intercalation into the negative electrode.

In the Nanohybrid Capacitor, Li4Ti5O12 is formed into nanoparticles so that the diffusion distance is decreased, thereby reducing the time required to react with Li ions to 1/2,000. At the same time, the use of carbon nanofiber increased the electric conductivity.

As a result, the new capacitor requires only 12 seconds for charging/discharging, in contrast to the existing Li4Ti5O12-based EDLCs that need about 1.5 minutes to one hour for charging/discharging. This is even shorter than the charging/discharging time (15 seconds) of the existing EDLCs that use activated carbon for the negative electrode.

For the production of the negative electrode, the nanoparticulated Li4Ti5O12 and the carbon nanofiber are subjected to a centrifugal process so that Li4Ti5O12 is evenly mounted on the external and internal sides of the carbon nanofiber.

Safer use, longer life

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